Plywood canoe

Discussion in 'Wooden Boat Building and Restoration' started by latestarter, Oct 2, 2012.

  1. rwatson
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    rwatson Senior Member

    Just a thought on this - I seem to remember you were trying to avoid anything but taped seams on your outside hull ?

    have i got this right ?
     
  2. latestarter
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    latestarter Senior Member

    My starting point rw was no cloth or tape.
    This may seem outlandish but the designer's website has this paragraph at the bottom of the page shown in the link,

    "Once the inside of the plank/panel chine joins has cured you can turn the hull over, clean up the chines on the outside, fill them with thickened epoxy and tape over them - note, some builders do not bother taping the outside seams (chines) except perhaps the centreline seam - this is fine for most applications unless the canoe is going to receive heavy use in which case the chine seams on the outside should be taped."

    http://www.makeacanoe.com/epoxying_&_taping.htm

    I understand he has reservations about this on the bigger designs.

    My thinking was, as an epoxy joint is stronger than plywood, why reinforce it. I now know epoxy tends to be brittle although I have lost a link to a guy who builds his motorboats with no tapes just fillets, he qualifies it by saying they are for his own use, he would tape them for others to avoid being sued. :)

    Regarding durability of the ply, in my day plywood dinghies survived well with just paint albeit with more maintenance.

    I have decided to tape the lowest chine on the outside as it is practically a butt joint and it will be hidden when the boat is upright. It is also the part most likely to get rubbed. The other chines meet at a good angle which is stronger when hit, as the force tends to be dispersed as compression in the planks instead of bending. I shall leave a decision on them to the end.

    LP I have confused you with terms I used. By keel I meant the flat panel made up of the planks in the first photo, these were glued length wise as in the photos in the previous post and then end to end.
    I do not have a photo of it on its own, the last photo shows it with the adjacent panels taped on. It provides a stiff lateral base for the rest of the hull.

    Just to emphasis my lack of experience with epoxy I have been using syringes and my biggest batch so far is 6ml of resin to 3ml of hardener. :eek:

    To help my thinking what weight of cloth do you use and how much resin do you take to wet it out. Does it take a second application to fill the weave and how much sanding prior to painting/varnishing?
     

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  3. pistnbroke
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    pistnbroke I try

    Quote
    ""Once the inside of the plank/panel chine joins has cured you can turn the hull over "

    Which implies you build it right way up .....as for joining and painting the place to look for this is the construction part of http://koti.kapsi.fi/hvartial/index.htm
    More commonly called Hannu's Boatyard
     
  4. rwatson
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    rwatson Senior Member

    The thing is, latestarter, that epoxy is brittle unless it is re-inforced with tape.

    The join between ply and epoxy is very strong if you join face to face, but the join to the ends can be problematic - which is why the tape is used.

    If you are only taping the inside , I cant understand all the angst about laying large layers of cloth. If you are only doing a full cover on the inside, its a piece of cake. You dont have to have one continuous layer for good looks, you can do multiple runs with joins at convenient places if you overlap 300mm or so.
     
  5. LP
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    LP Flying Boatman

    Latestarter,

    It's pretty typical to use 6 oz. cloth for sheathing a vessel like this one. Sorry, I didn't convert to grams and meters. The 6 oz. is per square yard. I think you can plan on close to that per square yard in epoxy usage or a little less. I'm surprised it is a straight butt joint for the keel panel. Maybe I misread your posts. Are there any bulkheads or frames in your build. I ask because I'm concerned about a straight butt with no other reinforcement, especially on the bottom panel. Maybe, I'm being overly cautious. As a suggestion, because I know your hessitant to sheath the hull, you might consider sheathing only to bottom to slightly past the first chine. Essentially, to the line where your chine tape would stop. Just my two pennies. Good to see you making progress.

    ------------------------------------------

    And with regard to filling the weave:

    This is the part where epoxy can be tedious. With 6 oz., 3-4 coats with just a light sand between coats. I try to plan my epoxy sessions about 4-6 hours apart so that I can almost finish in a day. If the epoxy is still green, you will get you molecular bond without the need for giving the previous coat some "tooth". My light sand between coats is to knock off the high points; bits of epoxy and glass that are standing proud. Some have metal scrapers for this task. You have abouy three choices for tools for your fill coats. The brush: it puts the epoxy on the thickest and is good for horizontal surfaces, but is prone to runs on more vertical surfaces. The roller: spreads a medium amount of epoxy very uniformly. It picks up from wet areas and lays down on dry areas. The scraper: lays down the thinest coat of epoxy, but tends to lay it in low areas and scrap it from high areas. Takes the most coat to build thickness. Once the weave is filled, then you sand for smooth and fair. I throw on a couple of more coats after this to insure I've not exposed any glass in the sanding process.

    On the inside of my boats, I will only wet out the glass once with maybe one fill coat. With a continuous layer of glass, it's appearance is acceptable and it provids texture for anti-skid purposes.
     
    Last edited: Oct 14, 2012
  6. latestarter
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    latestarter Senior Member

    pistnbroke, interesting link you provided, only had a chance to look at cartopping so far.
    I have never disputed that right way up is the normal way to build, but it would not work using adhesive tape see photo. The link for the full manual is http://www.makeacanoe.com/On Line Stitch and Tape Manual.htm

    LP your suggestion of covering the bottom and first chine has appeal, it would mean only 2 edges to be faired instead of 4.
    I am confident about the bottom panel, the longitudinal joint will be covered by a 4 inch wide butt strap on the inside, any join in the strap will not coincide with the middle cross butt joint and there will be a rubbing strip epoxied on the outside of the longitudinal join. The butt joints in the middle of the boat on all the panels are reinforced with tape inside and out.

    It will be awhile before I need to take any decision on taping and clothing etc.
     

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  7. LP
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    LP Flying Boatman


    Good deal. I thought there might be more to the story.
     
  8. Dhutch
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    Dhutch Junior Member

    Very interesting thread this. Im considering making a plywood boat (little larger, but same construction) and as said the disagrement or or differenet methods are all interesting.

    Daniel
     
  9. txriverrat
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    txriverrat Senior Member

    I am going to run my jaws here a little bit. Dang we are building a boat not a space ship Lol
    I have built 15 s and g yaks and canoes and use them in a rough rocky fast water inviroment.

    Butt joints, they hold great and the only reason for the tape is to make them tough enough to hold together till you get the boat glassed. Butt them up run your epoxy slap some tape on it and then do the other side ,now you can handle the strip with little fear of it snapping.
    14 boats have been built upside down ,one boat sitting upright , that is the only boat that isnt fair on the bottom. I want ever build another up right , I use two saw horses set close to each end leveled sideways between those and my internal frames it is easy to line up the hull. I have used this method for boats using 1/8 ply 1/4 ply and strips ,it works well.
    Example
    [​IMG]

    Laying the cloth can be done several ways ,all in one piece or in two pieces with a 3 inch over lap in the center for extra wear protection. full length is no problem
    [​IMG]
    Lay the cloth on the outside of the hull every few minutes start at the center and take your hand and smooth the cloth going out to the ends do this several times and the cloth will relax and conform to your hull.
    I apply epoxy with a roller starting at the center and working tward each end,then come back with a good quality brush every 30 minutes and dry brush it out .

    [​IMG]
    I dont use fast epoxy so there is plenty of time to brush it,and you dont have to worry about the blush.
    This is just what works for me.
    hope it helps
    Ron
     
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  10. latestarter
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    latestarter Senior Member

    That does help, Ron.
    Does that give you a final finish or do you need extra coats to fill the weave etc. ?
    How much epoxy do you use in total for the cloth?

    I have made some progress, the chines on the middle 8 feet of the hull are finished.
    The 2 chines that are angled were straight forward, having used the West six10 to tack the inside, the adhesive tape could be removed which gave a clear run for the filleting. The angled planks also gave good access to prime the edges before filleting. The only slip up was the adhesive tape on the inside to stop the epoxy leaking from the joint had a wrinkle which allowed some fillet through.
    The chine next to the flat bottom was more of a problem. I was wanting a gap between the planks to seal and prime the edges before filleting what is in effect a butt joint. Where the planks were in contact they could be aligned with the adhesive tape, elsewhere the flat bottom tended to sag below the next plank.
    I used a combination of props and weights to bring the edges into alignment then filleted short lengths. Having filleted the parts where there were gaps, I used a drill to make enough room on the chine to insert a jigsaw blade then ran it along the chine to the next fillet. This gave a gap about 1.5mm wide which was enough to prime the edges using a small artist's brush.
    I usually epoxied in the afternoon and the following morning removed the masking tape and shaped the fillets with the surform and finished with 80 grit aluminium oxide paper. It turned out better than I expected. I am now working on getting the shape of the ends right.
     

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  11. txriverrat
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    txriverrat Senior Member

    on a boat that size 1 1/2 gallons of epoxy should do it ,that is for me now a new builder it may be different. You are putting a lot of time and effort into this build ,go ahead and cover inside and out with epoxy and cloth,you will be glad you did in the long run. It will take several coats to fill the weave.
    You dont reach maximum strength till the weave is filled and one coat on top of that.
    Final finish for me spar varnish over the finished boat , it is hard gives a good shine and covers any finish sanding you need to do on the epoxy and the big one it gives uv protection to the boats.
    I use plastic ties for the biggest part of my builds ,I have used wire ,tape , hot glue , ropes ,workout weights , anything you need that works it ok. For spacing the edges and helping to fair the hull I use craft sticks (popsicle sticks)
    to space the hull if I need them

    Just a couple suggestions , I always wipe down the wood where I have applied tape with acetone before I apply my cloth. And one your first build dont get to hung up on weight ,the difference between a 45 lb hull and a 47 lb hull you will never notice but that extra 2 lbs may make a world of difference in the longevity of your hull.
    Boat is looking great.
    Ron
     
  12. txriverrat
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    txriverrat Senior Member

    oh forgot this, I never sand between coats of epoxy unless it is over 72 hours between coats. The sanding dust gets in the weave and is hard to get out. After I fill the weave I then sand off any rough spots before putting my final coat on.
    Ron
     
  13. rwatson
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    rwatson Senior Member

    "You dont reach maximum strength till the weave is filled and one coat on top of that."

    Not so - as soon as all the weave is saturated, you have reached maximum strength. The other layers will add protection and prevent moisture into the wood, but its not a structural thing. Often, if weight is critical, builders will leave areas that wont get any wear or water without the final topcoat.

    "I always wipe down the wood where I have applied tape with acetone before I apply my cloth"

    This is very helpful, but the manuals emphasize lots of water and detergent. Acetone will dissolve any greasy film from 'blush' and spread it all over the hull. This will be washed off with strong detergent and water.

    "I never sand between coats of epoxy unless it is over 72 hours between coats. The sanding dust gets in the weave and is hard to get out. After I fill the weave I then sand off any rough spots before putting my final coat on."

    This is the recommended method alright. Its a nuisance to have to hang around waiting for the coats to go off, so you can apply the next coat. This "72 hours" gets quoted a lot in literature, but I haven't come across this in the epoxy manuals. West systems say "Make sure the previous coat has cured firmly enough to support the weight of the next coat. To avoid sanding between coats, apply all of the coats in the same day. "

    http://www.westsystem.com/ss/barrier-coating/
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    - that can also be done to provide an anti-slip finish where desired like a deck or cockpit floor.
     

  15. txriverrat
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    txriverrat Senior Member

    I dont want to start a heated discussion here but I will stand by the statements I made

    "You dont reach maximum strength till the weave is filled and one coat on top of that."

    this is correct , a little back ground here I owned part of a small boat and fiberglass company for a few years and dealt with the engineers and chemist quite often.
    Note the use of maximum strength ,there is no way that a piece of cloth without being filled and a cover coat applied is as strong , now this comes from folks that make the glass and epoxy and it just makes sense ,when you run those extra coats filling the weave it makes a solid straight line unit which is always stiffer than a crooked line.

    "I always wipe down the wood where I have applied tape with acetone before I apply my cloth"
    We are talking tape on raw wood ,the only epoxy you have is in the fillets so blush is a non factor ,plus soap and water will not dissolve a lot of adhesives that are used on tape

    I never sand between coats of epoxy unless it is over 72 hours between coats. The sanding dust gets in the weave and is hard to get out. After I fill the weave I then sand off any rough spots before putting my final coat on."
    Those same engineers told us we could coat epoxy within 72 hours and still get a chemical bond
    Leaving the weave open does give you a rougher finish ,I agree with that. but it isnt as strong ( maybe it is strong enough) it want take near as much wear to get into your cloth and leave white spots from the glass showing and the one I hate the most get mud inside that boat and into the weave and it just about takes scrubbing with a toothbrush to get it out.
    Ron
     
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